The present invention relates to an antenna apparatus used to send and receive radio signals. Specifically, the present invention relates to an ultra wide band, small-size antenna apparatus capable of being applied to a wireless communication system which uses ultra wide band frequency bands such as ultra wide band communication for transmission and reception.
In more detail, the present invention relates to an antenna apparatus according to the patch antenna scheme configured by disposing a radiating conductor and a reference conductor (ground conductor) opposite to each other through an intermediate of insulative substance. Furthermore specifically, the present invention relates to a thin microstrip patch antenna having unidirectivity in a wide band.
In recent years, there is an increasing demand for wireless LAN systems in accordance with their increased processing speeds and reduced prices. Today, especially, introduction of a personal area network (PAN) is taken into consideration for the sake of information communication by constituting a small-scale wireless network between a plurality of electronic devices around a person. For example, there are provided different wireless communication systems using frequency bands such as 2.4 GHz and 5 GHz bands that need not be licensed by governing legal authorities.
Recently, particular attention is paid to “ultra wide band (UWB) communication” as a wireless system for short-distance, ultra-high speed transmission. The UWB communication is expected to be commercially available. The UWB communication system is designed for wireless communication to send and receive data by diffusing it to an ultra-wide frequency band from 3 GHz to 10 GHz, for example. Presently, the IEEE802.15.3 working group and the like are discussing access control systems for the ultra wide band communication.
Wireless communication including wireless LAN uses antennas for information transmission. Various types of antennas are available. In particular, wide band antennas can be used for the UWB communication to send and receive data by diffusing it to an ultra-wide frequency band. Small antennas contribute to miniaturization and light weight of wireless devices.
For example, a microstrip patch antenna is known as a thin antenna. That is, this antenna apparatus is configured by disposing a radiating conductor and a reference conductor opposite to each other through an intermediate of insulative substance. Generally, the radiating conductor is shaped to be rectangle or circular, although not specifically specified. The insulative substance is inserted between the radiating conductor and the reference conductor and is as thin as approximately one tenth of a wireless frequency wavelength or smaller. Accordingly, the microstrip patch antenna can be configured to be very thin. In addition, the microstrip patch antenna can be relatively easily manufactured by etching a double-sided copper clad insulative substrate. That is, the microstrip patch antenna is characterized by being relatively easily manufactured or being easily integrated with the circuit substrate.
FIG. 7 shows a configuration example of the microstrip patch antenna and its impedance matching circuit. The radiating conductor is shaped to be rectangle as shown in FIG. 7 or to be circular. A dielectric material is used for the insulative material and has the thickness of approximately one tenth of the wireless frequency wavelength, i.e., very thin. Actually, the microstrip patch antenna is often manufactured by etching a double-sided copper clad insulative substrate. Accordingly, the microstrip patch antenna is easily manufactured or easily integrated with the circuit substrate.
When the microstrip patch antenna having the above-mentioned construction is excited in the lowest mode (or TM10-mode for the rectangular antenna), the antenna indicates the unidirectional radiation directivity approximately along the z-axis direction. There is available directional gain of approximately several dBi. Because of the excitation, the feeding point is provided at a position with a slight offset from the center. Adjusting the offset length makes it possible to align to 50 ohms.
The microstrip patch antenna itself uses a narrow operable band. The microstrip patch antenna is unsuitable for PAN systems and the like that use wide bands as operable bands. Depending on design parameters, a bandwidth smaller than or equal to VSWR2 exhibits the level of approximately several percentages. This drawback considerably has limited an applicable scope.
On the other hand, there is provided a wide-band antenna apparatus including a reference conductor and a radiating conductor connected by a feeding wire to supply power. The reference conductor and the radiating conductor are disposed so that at least parts thereof face to each other. An intermediate substance is placed between opposite parts of the reference conductor and the radiating conductor. At an operative wireless frequency, the intermediate substance indicates the conductivity approximating to greater than or equal to 0.1 and smaller than or equal to 10. Consequently, the antenna apparatus can provide the sufficient gain in a wide band. The reason follows. There is provided the substance having the conductivity approximating to greater than or equal to 0.1 and smaller than or equal to 10 between the reference conductor and the radiating conductor. The substance characterized by such conductivity can properly cause a signal leak between the reference conductor and the radiating conductor (e.g., see patent document 1).
Further, there is provided a thin wide-band antenna apparatus including a reference conductor and a radiating conductor connected by a feeding wire to supply power. The reference conductor and the radiating conductor are disposed parallel and opposite to each other near a pole. A magnetic material is placed between the reference conductor and the radiating conductor opposite to each other. At an operative wireless frequency, the magnetic material has relative magnetic permeability greater than 1 and approximately smaller than or equal to 8. Consequently, the antenna apparatus can provide the sufficient gain in a wide band (see Japanese Patent Application Laid-Open No. 304115/2003).
FIG. 8 shows results of simulating VSWR characteristics to exemplify comparison between operative bandwidths using a dielectric material and a magnetic material as insulative materials placed between the reference conductor and the radiating conductor opposite to each other. The bandwidths are compared by appropriately adjusting the relative permittivity or the relative magnetic permeability so as to keep the same antenna size. When the dielectric material with relative permittivity 3 is used, the operative bandwidth indicates 6.5% (VSWR of less than 2.5). When the magnetic material with relative magnetic permeability 3.6 used, the bandwidth indicates as high as 21.2%.
When the magnetic material is used as an intermediate, the impedance matching may be difficult by simply adjusting the offset length at the feeding point. Such case can be solved by using an impedance matching circuit as shown on the right of FIG. 7, for example.